Gas turbines utilize a compressor for compressing air which is mixed with a fuel and channeled to a combustor. The mixture is ignited within a combustion chamber in the combustor from which hot combustion gases are generated. The combustion gases are conveyed to a turbine, which extracts energy from the combustion gases for powering the compressor, as well as producing useful work to power a load, such as an electrical generator.
Conventional combustors typically include a combustor casing, a liner, a dome, a fuel injector, and an igniter. The combustor casing operates as a pressure vessel containing the high pressure inside the combustor. The liner encapsulates a combustion zone and may be used to manage various airflows into the combustion zone. The dome is the component through which the primary air flows as it enters the combustion zone. A swirler may be used in association with the dome. The dome and swirler provide the function of generating turbulence in the flow to mix the air and fuel. The swirler may create turbulence by forcing some of the combustion products to recirculate.
Combustors are designed to first mix and ignite the air or an oxidizing fluid and fuel, and then mix in more air to complete the combustion process. The oxidizing fluid may be an oxidizer such as air, or a mixture of an oxidizer and a diluent such as water, steam, Nitrogen or other inert substance used to dilute the oxidizer. Design criteria for combustors include a number of factors, such as containment of the flame, uniform exit temperature profiles, range of operations and environmental emissions. These factors affect turbine reliability and power plant economics.
During the operation of a gas turbine combustor, instabilities may occur when one or more acoustic modes of the system are excited by the combustion process. The excited acoustic modes may result in periodic oscillations of system properties (e.g., velocity, temperature and pressure) and processes (e.g., reaction rate or heat transfer rate).
Combustion instabilities may result from flame sensitivity to acoustic perturbations. The perturbations disturb the flame, causing heat release fluctuations which in turn generate acoustic waves that reflect off combustor surfaces and re-impinge upon the flame, causing additional heat release oscillations. In some situations a self-exciting feedback cycle may be created. This feedback cycle results in oscillations with large amplitudes.
Another source of combustion instabilities may be oscillations in the fuel/air ratio in premixed combustors. Pressure fluctuations in the premixer may cause an oscillating pressure drop across the fuel injectors, resulting in an oscillatory delivery of fuel to the combustor. These create further flow and pressure disturbances in a feedback loop. This mechanism may be self-exciting when the product of the frequency of oscillation, f, and the delay between the time a fuel parcel is injected into the premixer and burned at the flame (premix time or Tau), are within a range of values. Tau is a function of the air velocity in the premixer and the premixer length.
Combustion driven oscillations negatively impact the life of gas turbine components which may result in more frequent outages and the de-rating of turbine power output. Additionally, combustion driven oscillations may also result in an increase of pollutant emissions (e.g. NOx and CO). Conventional combustors exhibit damaging combustion driven oscillations within their operating range and are sensitive to fuel-injection pressure ratio (modified Wobbe), combustor loading and inlet conditions.